1. Field of the Related Art
The present invention concerns a photothermographic material and an image forming method using the photothermographic material.
2. Description of the Related Art
In recent years, there has been a strong demand to decrease the volume of liquid processing wastes in the medical field both for environmental protection and economy of space. In order to use photothermographic materials in medical diagnosis and photographic techniques they must be capable of being exposed efficiently by laser image setters or laser imagers; further the materials must be capable of forming clear black images having high resolution and sharpness. With photothermographic materials, thermal development processing systems can be used that eliminate the use of solution system processing chemicals, have a more simple construction and do not deteriorate environments.
While such requirements exist also in the field of general image forming materials, in medical imaging, high image quality with excellent sharpness and grainin are particularly required since fine expression is needed; blue black image tone are preferred to facilitate diagnosis. At present, various kinds of hard copy systems that utilize pigments and dyes such as ink jet printers or electrophotography have been marketed as conventional image forming systems, but they are not satisfactory as image output systems for medical use.
Thermal image forming systems utilizing organic silver salts are described in the literature. In general, a photothermographic material has an image forming layer and the image forming layer contains a catalytically active amount of photocatalyst (for example, silver halide), a reducing agent, a reducible silver salt (for example, organic silver salt) and, optionally, a color toning agent for controlling the tone of silver dispersed in a binder matrix. The photothermographic material, when heated to a high temperature (for example, 80° C. or higher) after imagewise exposure, forms black silver images by oxidation/reduction reaction between a silver halide or reducible silver salt (functioning as an oxidizer) and a reducing agent. The oxidation/reduction reaction is promoted by the catalytic effect of latent images of the silver halide formed by exposure. Accordingly, black silver images are formed in an exposed region.
Fuji Medical Dry Imager FM-DPL has been sold as a medical image forming system using the photothermographic material.
Production of the thermal image forming systems utilizing the organic silver salt includes a method of production by solvent coating, and a method of production by coating and drying a coating solution containing, as an aqueous dispersion, with fine polymer particles as a main binder. Since the latter method does not require steps such as recovery of the solvent, production facilities are simple and it is advantageous in mass production.
Since the image forming system utilizing the organic silver salt described above has no fixing step, there is a significant problem of worsening print out, particularly when light is applied. U.S. Pat. No. 6,143,488 and EP-No. 0922995 disclose methods of utilizing AgI formed by conversion of organic silver salts as a means for improving the print out. However, no sufficient sensitivity could be obtained by the method of converting organic silver salts with iodine as described therein and it was difficult to obtain an actual system.
In addition, photosensitive materials using AgI are disclosed, for example, in International Publications Nos. 97-48014 and 97-48015, U.S. Pat. No. 6,165,705, and Japanese Patent Application Laid-open (JP-A) No. 8-297345 and JP No. 2785129 but none of them could attain a sufficient sensitivity/fog level and was not durable for practical use as laser exposure photosensitive materials. While JP-A No. 2000-305213 discloses an image forming method using blue-ultra violet laser light and photosensitive material, it does not use AgI and is insufficient in view of the sensitivity.
Therefore, there is a need in the art for the development of improved photothermographic materials that suppress the occurrence of fogging or print out while not lowering the sensitivity.